first of all i am very thankful to all those who give very important info for increasing pr.
now in my views good conmtent and unique backlinks are the main cause of increased pr. also avoide spamming.

If you want to Increase PR of your site than generate good quality one way backlinks for your site from high PR pages of your niche. Before that, concentrate on making a good site, with plenty of good content. If you're selling stuff, make sure you have well written content about each product. Make sure that all links within your site are HTML links. Simultaneously link your site to .edu, .gov sites. These are the only and the best way to Increase PR of your site.